Flying Notes/Info

During some of my summer flights in particular, my Garmin GPSMap 296 showed altitudes hundreds of feet higher than the panel altimeter, also observed by my non-pilot friends in the aircraft with me.

I attributed most of the difference to non-standard temperature (heat-related) density effects, however the GPS unit indicated we were a few times near class B airspace floors when the panel altimeter indicated we were plenty low enough below it.  So I hit the books and here's what I found:

(a) Fly by the altimeter, not the GPS, for cruise and airspace altitude selection - as long as you have a valid altimeter setting per FAR 91.121.  For now at least, the FAA expects you (and all other pilots) to refer to the barometric altimeter in the aircraft for your cruise and airspace altitude selection decisions, so long as safe terrain clearance is observed.

This guarantees all planes' altimeters in a given vicinity experience the same conditions, and therefore the same errors, because barometric altimeters are quite precise even though uniformly inaccurate from local factors (ATIS/ASOS/AWOS settings, altitude, heat, humidity).  Consequently, collision risks are mitigated because of the FAR-mandated common reference is maintained for standard altitude selection procedures.

(b) Terrain avoidance with GPS is generally better - though it depends upon which GPS equipment you are using. Most current aviation GPS units display 'virtual' MSL altitudes by taking the local EGM96 geoid corrections into account, and with WAAS are both accurate and precise. (See the discussion of GPS Aircraft Altitude Accuracy below.)

(c) Jedi flying trick: "ATC this is not the altitude you're looking for..." - My (VFR) GPS could even indicate we are in an airspace altitude bust condition but if the barometric altimeter says otherwise, we are fine so long as we have the right setting. The mode C transponder tells ATC what your altimeter reads and if the Kollsman window setting complies with local ATIS/ASOS/AWOS, etc., ATC 'sees' what your altimeter indicates.

(d) GPS WAAS with barometric altimeters - Stanford University published a FAA-sponsored research paper to examine tradeoffs and confidence margins of using 1, 2, or 3 WAAS frequencies with and without a barometer. A review of barometric altitude computation is included.

(e) AviationBanter.com thread on this topic - I found this thread after I wrote up most of these two GPS altitude-related notes.

When the FARs update in the future to refer to GPS altitudes, we will still have these issues to content with, but in reverse, because a barometric altimeter will be kept as a backup to the GPS or built into it, as some GPS units already do.

• GPS Aircraft Altitude Accuracy

Were those observed altitude differences between my GPS and the barometric altimeter explained entirely by density effects and local weather?  Back to the books and not a little Google searching and cross-checking.  Here's what I learned: Mostly yes.

With WAAS, GPS altitude errors have been within 2 or 3 meters (6-10 ft). But GPS altitude errors are unrelated to altitude standard of reference concerns:

(a) FAA aviation charts and airways refer to Mean Sea Level (MSL), the 19-year-(Saros cycle)-tidal-average local sea level, also called the orthometric reference datum.

The NASA/NIMA EGM96 geoid reference model closely approximates MSL to within a few feet in most cases (or at most +66/-132 feet [Harvard]) using an order-360 and degree-360 spherical gravimetric potential harmonic model having over 130,000 coefficients.

As such, MSL and the EGM96 geoid datum are close enough to be considered the same in most cases (see FAA Order 8260-48 RNAV Approach Construction, 1.4.10).  So here I'll call the EGM96 geoid reference 'virtual' MSL, or vMSL for short.

(b) However most U.S. GPS device calculations internally reference the more perfect and abstract WGS84 reference ellipsoid datum. In the San Diego/Los Angeles area, the EGM96 geoid (vMSL) is about 105 feet below the WGS84 ellipsoid.  In other places EGM96 and WGS84 can differ by more than 300 feet [NASA/JPL].

Source: NASA JPL

(c) Most GPS units display vMSL altitudes by interpolating a stored table of EGM96 geoid heights above the WGS84 ellipsoid.  Garmin IFR models 430/530 and 480 convert their internal WGS84 altitude to vMSL using the EGM96 geoid datum model.

(d) When I checked the default datum in my Garmin GPSMap 296 (VFR), sure enough, it was WGS84. I would have thought this means to find the vMSL altitude in my area, I need to add 105 feet per (b) above, to the unit's displayed altitude.

However, my GPS unit appears to display vMSL atlitudes.  If my GPS displayed WGS84 altitudes, I should expect the unit to show about 427 MSL elevation - 105 ft geoid height = 320 feet altitude on the ground at MYF airfield.  When I performed an on-field test with WAAS correcting 11 GPS satellites, my GPSMap 296 reported the MYF field elevation as 415 to 420 feet, +/- 8 ft, pretty much the correct MSL elevation.

(e) AviationBanter.com Thread on this topic - I found this thread after I wrote up most of these two GPS altitude-related notes.

I'm not certain but I'm guessing it's because VFR GPS units do not have as extensive a geoid table or specific details for RNAV/VNAV approaches in particular that the FAA authorizes them only for VFR situational awareness, not as primary flight instruments.

But in an partial panel or IMC emergency it's a good resource to have for turn/bank information as much as for position and altitude - my GPSMap 296 has a simulated instrument panel that tracks quite well to compass, turn and bank.  But knowing a WAAS altitude is only a few feet off is still helpful.